As is well known, the power and efficiency of gas turbine engines typically increase with increasing nominal operating temperature, but the ability of the turbine to operate at increasingly higher temperatures is limited by the ability of the turbine components, especially the vanes and blades, to withstand the heat, oxidation and corrosion effects of the impinging hot gas stream and still maintain sufficient mechanical strength. Thus, there exists a continuing need to find advanced material systems for use in components that will function satisfactorily in high performance gas turbines, which operate at higher temperatures and stresses.
One approach to providing improved turbine components is to fabricate a strong, stable substrate having the shape of the component, and cover the substrate with a thin protective coating that resists the oxidation and corrosion effects of the hot combustion gas stream. The underlying substrates, usually nickel-base or cobalt-base superalloy compositions, were at one time formed by common forging or simple casting procedures, but now improved performance results from use of cooled airfoils made by directional solidification. Even greater operating temperatures are possible by casting the substrate as a single crystal having no grain boundaries which might cause premature failure, and with the single crystal orientation selected to meet required creep-rupture and fatigue lives.
Insulative coatings further enhance turbine performance by reducing heat transfer into cooled airfoils, thereby reducing the requirement for cooling air, which is a performance penalty. Durability of turbine components is also enhanced by insulative coatings that minimize metal temperatures and thermal stresses in the superalloy component.
Coating systems for insulating gas turbine engine components typically include several layers of differing compositions and properties in order to provide an optimal combination of benefits. For example, one layer may be relatively thick and porous to provide an insulating effect but, by itself, offer little resistance to oxidation, erosion, and corrosion. Ceramic materials are the predominant choice for this insulating layer, referred to as a thermal barrier coating (TBC). The outer surface of such a layer may be protected from erosion by providing a thin, hard, dense surface layer.
Generally, a thin metallic layer, termed a bond coat, is applied on the substrate to adhere a ceramic insulating layer and protect the substrate through formation of an adherent oxide scale, such as a layer of aluminum oxide (alumina), which resists the oxidizing effects of the hot combustion gas stream. Other elements present in the bond coat contribute to the ability of the protective ceramic coating to adhere to the substrate through many cycles of gas turbine startup and shut down.
The service life of a coating system as described above, conventionally termed a thermal barrier coating (TBC) system, is typically limited at high temperatures due to excessive growth of the oxide scale on the bond coat and flaws which develop within the interfacial zone between the metallic bond coat and insulative ceramic layer. Thermally-induced deterioration of the interfacial zone coupled with stresses induced by thermal transients, ceramic-superalloy thermal expansion mismatch, and oxide growth eventually leads to spalling of the insulative ceramic layer.
It should be apparent from the foregoing general discussion of the art that further improvements in both the effectiveness and useful life of thermal barrier coating systems are required in order to survive the increasingly severe operating conditions in high-performance gas turbine engines. Therefore, a general desire is to provide methods for improving the spallation-resistance of thermal barrier coating systems for gas turbine engine components by enhancing the adherence of the insulative coating to the bond coat.